Frequency Modulation (FM) is a form of modulation in wireless communication which represents information as variations in the instantaneous center frequency of a carrier wave. Digital data can be transmitted by shifting the carrier frequency among a set of discrete values, a technique known as frequency-shift keying. FM requires a wider bandwidth than amplitude modulation by an equivalent modulating signal, but this also makes the signal more robust against noise and interference. Frequency modulation was chosen as the modulation standard for high frequency. A plurality of FM frequencies (channels) each separated by a frequency spacing may be broadcasted by a transmitter tower or a radio station.
A FM receiver of a FM radio includes a tuner with a tunable local oscillator (LO) to scan or search for frequency channels. Scanning may be performed by tuning the LO across the full tuning range of the LO or sweep the LO back and forth over a narrower tuning range to search for a signal of interest such as a FM channel. A FM channel may be detected or tuned if the FM receiver can successfully process a signal of sufficient signal amplitude; and/or the tuner is able to establish an intermediate frequency (IF) signal that is substantially the same or close to a defined offset of the FM receiver. When signals of two same frequencies (from different broadcast stations) are received by the FM receiver, the FM receiver may process the stronger of two signals being broadcasted on the same frequency.
Received signal strength is indicated by Received Signal Strength Indication, RSSI. RSSI is directly known to users of wireless networking of IEEE 802.11 protocol family and may be measured in the IF stage before the IF amplifier. The RSSI output is often a DC analog level. It can also be sampled by an internal ADC and the resulting codes available directly or via peripheral or internal processor. If a signal level is above a detection threshold, the logic level of RSSI may be set to high, otherwise RSSI may be set to low.
Sometimes the broadcasted channel frequencies may drift due to old transmitters lacking frequency stability or due to equipment defect. A tuning error may occur resulting in unsuccessful channel detection if the channel frequency drift exceeds certain acceptable range. Likewise frequency drift in the LO of the FM receiver may cause a failure in channel detection. Using high stability frequency sources in the transmitter and/or the FM receiver may improve channel detection success. Frequency source stability may be improved where the output frequency source is locked to a highly stable reference using a feedback circuit. The principle of phase locked loops is well known in the art and its discussion is beyond the scope of the invention.
FM signals may be broadcasted in the Radio Data System (RDS) or Radio Broadcast Data System (RBDS) standard format. The RDS data format may contain information such as alternate frequencies of the broadcast station, the clock time, program identification with known channel frequency, channel spacing, station ID, country code or country identity, regional links and Enhanced Other Networks (EON) etc. Portable electronic devices and wireless devices feature a high number of available channels. For example, a driver entering a new area served by different broadcast stations whose FM receiver may need to scan up to all available channels to locate a desired channel. Depending on the country and the channel spacing, the processing to a desired channel may result in a long delay.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.